This fascinating volume introduces the general public to the microbial universe, stressing the essential role microbes play in our daily lives. Its 24 chapters explain, in non-technical language, all the important principles of microbiology, the nature of microbial life, and how microbes interact with the environment and with other living organisms, including humans.

Through a lively historical approach, the author pays tribute to the pioneers of microbiology and biological chemistry who discovered the microbes responsible for diseases that were the scourges of man, animals, and plants, and ways to combat them; explained how the activities of many microbes benefit higher forms of life directly and through their effects on the biosphere; and devised procedures for exploiting microbes as tools of biotechnology for improvements in medical treatments, agriculture, waste disposal, and more.

Designed to educate a diverse readership, Microbes provides the background and basic information needed to develop informed opinions on issues affecting our health and the preservation of the natural biosphere.

Rational explanation of infectious disease and other manifestations of microbial life had to await two developments: acceptance of the concept that "invisible microbes" existed and tangible evidence of their reality. The first evidence that we are surrounded by multitudes of microbes was provided by observations made by Antonie van Leeuwenhoek with primitive microscopes in 1674. The historic discovery revealed not only the physical reality of living microbes, but also their diverse nature. These momentous advances, which are discussed in this chapter, illustrate one of the common characteristics of many waves of new discoveries in biological science, namely, the use of new or improved experimental techniques for making observations. Leeuwenhoek can be regarded as one of the great explorers of all time—indeed, he discovered a whole new world by examining an enormous range of natural samples. In the course of his studies, he described for the first time the sperm cells of animals, including humans, and he was also the first person to recognize that in the fertilization process, the sperm enters the egg cell. Leeuwenhoek’s observations were all described (in the Dutch language) in about 300 letters, 190 of which were addressed to the Royal Society of London.

In addition to the single-cell microbes that abound in nature, there are also various types of multicellular forms; these have more complex internal structures and reproduce in more complicated ways than the single-cell microbes. Yeasts, microalgae, and protozoa are types of single-cell eukaryotes that differ from one another in various ways, for example, in how they obtain energy for growth. Opposite are the prokaryotes, that is, bacteria. These organisms have a comparatively simple anatomy and do not have a distinct nucleus. The DNA of bacteria is the same kind found in other living organisms, but it is organized in a different fashion, rather like a fuzzy blob floating free in the cell interior. The prokaryotes are unicellular organisms and include the taxonomic domains Eubacteria and Archaebacteria. The term Archaebacteria implies that this domain includes organisms of great antiquity, but there is no evidence to support this notion. The Eukaryotes include all the rest of life on Earth except the Bacteria and Archaea. Among them are grouped the yeasts, fungi whose usual growth mode is unicellular. Fungi are multicellular eukaryotic microbes of great importance with respect to chemical change in the biosphere, infectious diseases of plants and animals, and production of antibiotics. Much of the contemporary molecular biology widely discussed in newspapers and news magazines is based on applying knowledge derived from the study of bacteria to the analysis of processes in plants and animals, including humans.

Following Leeuwenhoek’s work in the mid-1600s, a century and a half elapsed before microbes were in the news again. The concept that microbes might be agents of chemical changes in their environments was not appreciated until the nature of fermentation processes was clarified. This was accomplished by Louis Pasteur, who demonstrated that production of alcohol from sugar by certain microbes-"alcoholic fermentation"-was, as he put it, a "consequence of life without air". The idea that there were forms of life that did not require air (that is, oxygen gas) must have seemed strange to many in Pasteur's time. Lactic acid fermentation occurs not only in milk, but also in our muscles when we move or exercise. In both situations, the fermentation has the same function, that is, both lactic acid bacteria and muscle cells derive the same kind of benefit from breaking down sugars to the smaller molecules of lactic acid. In 868 Pasteur settled the vexing question of the cause of the alcoholic fermentation to his own satisfaction, concluding that "alcoholic fermentation is an act correlated with the life and organization of yeast cells". The air in the Earth's atmosphere contains about 20% oxygen gas, and one result of Pasteur's research was recognition of the important fact that there are many kinds of microbes that do not need this oxygen. He named such microbes anaerobes.

This chapter describes the most important chemical substances found in microbes. These substances are, in fact, the same classes of substances found in all types of cells. The terms molecules, carbohydrates, fats, proteins, DNA, are encountered daily in our lives: in newspapers, television advertisements, cereal box labels, and so on. The chapter examines some basic definitions and concepts of chemistry. A table in the chapter lists the elements of special importance in living matter, their relative weights (mass units), and their approximate abundance in the human body. The carbohydrates, fats, and proteins account for the major part of our foodstuffs, which we obtain mainly from plants and other animals. In cellulose, the glucose units are joined end to end, as in a linked chain. Glucose units can be joined together in other ways to produce polysaccharides with highly branched structures. There are two important examples of such large molecules (often referred to as macromolecules): glycogen, which occurs in animal muscle cells and in some microbes, and starch, which accumulates in certain plants. It is know that metabolism also involves interconversions of dietary components, for example, the transformation of carbohydrates into fats. The growth of microbes (as opposed to animals) rarely involves a digestive phase; rather, microbes are typically dependent on supplies of certain small molecules in their environment.

Animals that were not seen firsthand to be born by natural reproduction were considered by the ancients to come into the world through spontaneous generation, as the result of the combined action of heat, water, air, and putrefaction. Numerous natural philosophers and scientists occupied themselves for centuries with the question of spontaneous generation, and it was still a hot topic when Louis Pasteur came onto the scene in the 1870s. By that time, it was clear that spontaneous generation of mice, maggots, etc., was unprovable and extremely dubious, and the argument then shifted to microbes. Pasteur devised ingenious ways of proving that microbes also do not arise by spontaneous generation, but that they are produced instead from other microbial cells. One of the important procedures used in Pasteur’s studies was preliminary destruction of all living microbes in nutrient fluids by heating, typically by boiling solutions for 15 to 20 minutes. More recent studies conducted by scientists at the United States Geological Survey have demonstrated that large numbers of living microbes are carried over long distances in clouds of dust, for example, across the Atlantic Ocean from African deserts.

Microbes catalyze numerous environmental chemical changes while they are pursuing their own purposes (namely, the fabrication of new microbial cells). These effects could not be intelligently interpreted until methods were developed for separation of natural mixtures of diverse microbial types into “pure” strains. Only then was it possible to determine by deliberate experiment, in the laboratory, the capabilities of individual kinds of microbes. With this knowledge (still being acquired), analysis of natural events in which myriads of microbes participate became feasible. Experiments by Louis Pasteur and his contemporaries clearly indicated that the variety of microbes in our surroundings have many different properties. The method most widely employed can be used either with enrichment cultures or with natural mixtures of microbes. With the availability of a simple method for obtaining pure cultures of bacteria and other microbes, the library of microbes in captivity expanded rapidly and has continued to do so. Once a pure culture is available, it is possible to grow the microbe in quantity so that detailed studies of its various properties can be made. From such knowledge, we can assess its significance with respect to its interactions with plants, animals, and other microbes; its possible roles in chemical conversions that occur constantly on the Earth's surface; and its potential for use in biotechnology.

At the time the agar-streaking procedure was introduced (and for the following 30 years), little was known about the specific nutrient requirements of different types of microbes. With the passage of time, it became evident that in many instances the chemical activities of bacteria and other microbes can be significantly affected by the presence or absence of particular nutrients in the growth medium. Since there is an extraordinary degree of metabolic versatility in the microbial world, thousands of culture media recipes have been proposed and used. For purposes of illustration, this chapter considers one important bacterium that is widely used in microbial research and happens to have relatively simple nutrient requirements, namely, Escherichia coli. Microbiologists in various countries were busy isolating pure cultures of microbes from soil, airborne dust particles, natural waters, and plant and animal surfaces. Cells of the standard (“type”) strain could be kept alive, but not growing, in the form of colonies or streaks on agar plates stored in the refrigerator at 4°C (39°F). An alternative procedure involves storage of cells at much lower temperatures. The American Type Culture Collection (ATCC) is the largest collection in the world and contains the most diverse assortment of known microbes. The scientific staff of the ATCC is constantly engaged in research aimed at improving cell preservation techniques and increasing knowledge of the properties of different species. A common technique now used for preserving microbial cells is based on freeze-drying (“lyophilization”).

Heating effectively kills virtually all kinds of microbes, but on occasion it is found that some can survive this harsh treatment. Except for certain “extremophiles,” the microbes that survive exposure to highly elevated temperature are usually species of bacteria that are able to produce specialized structures called “endospores.” Some species of disease-producing bacteria form endospores, and their long-term persistence in soils or other natural reservoirs can pose public health problems (for example, anthrax is caused by the endospore-forming bacterium Bacillus anthracis). Endospores—referred to hereafter simply as spores—are formed mainly by three genera, commonly found in soil: Bacillus: rod-shaped aerobes, Clostridium: rod-shaped anaerobes, and Thermoactinomyces: aerobic bacteria that grow best at slightly elevated temperatures (50№C). Although clostridial spores are very resistant to heat, they can be killed by appropriate heating procedures—for example, by superheated steam under pressure. Cryptobiosis, or “latent life,” is defined as the state of an organism when it shows no visible signs of life and when its metabolic activity becomes hardly measurable or comes (reversibly) to a standstill. A number of studies have shown that spores of Bacillus anthracis and Clostridium tetani can remain viable for at least 50 to 70 years. Bacteria of the genus Thermoactinomyces produce heat-resistant spores and grow optimally at a temperature of about 50°C (122°F). They are present in most soil samples and sporulate profusely in habitats such as compost, haystacks, and stored cereals.

Microbes are prominent agents in the recycling of several major chemical elements on Earth, notably oxygen, carbon, nitrogen, and sulfur. The microbes decompose organic matter to obtain energy and/or nutrients for their own multiplication in several ways including fermentation. Numerous species of microbes are engaged in this phase of the carbon cycle, which results in the conversion of organic carbon to CO2. This chapter examines how carbon moves through the cycle. In contrast to animals, which require organic compounds of carbon, plants grow on CO2, the major form of inorganic carbon. The utilization of CO2 by green plants through photosynthesis is the largest chemical process on Earth. Microbes are important agents of much of this carbon atom traffic, which is illustrated in a diagram that indicates only the general outline of the carbon cycle and introduces two new terms: autotroph and heterotroph. Shut off from sunlight, the plants died, and their decomposition was greatly slowed due to shortage of O2 in the muds. The organic matter was consequently converted to peat (partly decayed vegetation), some of which was further transformed to coal. In other words, coal deposits represent huge amounts of modified organic plant materials that have escaped the dynamic carbon cycle. When we burn coal and thereby convert it to CO2 we are accelerating return of carbon to the active cycle. Combustion of oil also restores CO2 to the atmosphere, and there is considerable evidence indicating that oil deposits were formed, in part, by ancient microbial processes.

The formation of methane (CH4) is actually the last phase of the anaerobic decomposition of photosynthetically produced organic matter by a large assortment of microbes with different appetites. The final microbes in this so-called food chain are the methanogens, a group of very anaerobic bacteria that generate methane. Methane (also known as "swamp gas") is colorless, odorless, combustible, and highly explosive in the presence of oxygen. Since methane can seep out of the ground through underground fissures near sewer and other pipes, it sometimes is the cause of dangerous explosions in neighborhoods adjacent to landfills. The digestive tracts of termites contain large numbers of methanogens, other anaerobic bacteria, and protozoa; these microbes efficiently process great quantities of wood and other biomass. Methane is already being used in about 400,000 vehicles around the world, including 250,000 in Italy and 20,000 to 30,000 in the United States. An engine designed especially for methane has an energy efficiency greater than that of ordinary automobiles. Microbes that can use methane or methyl alcohol (CH3OH) as their sole source of carbon and energy are widely distributed in nature (in mud, natural waters, and soils). Organisms with this capacity are called methylotrophs because the common chemical feature of methane and methyl alcohol is the methyl group. Methane utilization is restricted to certain bacterial species, which frequently can also grow on methyl alcohol. Imperial Chemical Industries in England has marketed dried cells of methylotrophic bacteria, grown on methyl alcohol, under the name "Pruteen."

The carbon cycle is intertwined with another major element cycle, namely, that of nitrogen. When the organic matter of dead organisms is mineralized by the actions of heterotrophic microbes, the nitrogen of proteins and nucleic acids is released in the form of ammonia. This is known as "ammonification" and is indicated as one phase of the nitrogen cycle depicted in this chapter. It was noted earlier that nitrate is an excellent nitrogen source for plant growth, and consequently, microbial production of this form of nitrogen in soils (nitrification) is of agricultural significance. Ammonia is as good as nitrate in providing nitrogen for plant growth, and the natural process of bacterial nitrogen (N2) fixation represents the possibility of a virtually inexhaustible supply of ammonia and useful organic nitrogen compounds. Nitrogen fixation is defined as the conversion of gaseous (atmospheric) N2 to ammonia and organic nitrogen and is the “final” phase of the Earth’s nitrogen cycle we will consider. Obviously, N2 fixation has been playing an important role in the recirculation of nitrogen atoms on Earth for millions of years. Nitrogenase consists of two enzyme proteins that act in concert; both of them contain metals that are essential for their enzymatic activities. The only unique aspect of N2 fixers is the transformation of N2 to NH3. Many genera of bacteria include species that are free-living N2 fixers, organisms that live and grow as unicellular forms in soil, natural waters, and other habitats. Nitrogen fixation is also widespread among genera of the cyanobacteria.

When plants and animals die, organic sulfur compounds are decomposed by bacteria with the release of hydrogen sulfide (H2S), an inorganic form of sulfur with an obnoxious smell. Sulfur occurs on the Earth in several other inorganic forms, all of which are constantly being interconverted on a massive scale. Bacteria are active agents in most of these processes, and it is not an exaggeration to say that bacteria "spin" the sulfur cycle. This chapter considers a simplified version of the sulfur cycle, paying particular attention to the three inorganic forms: sulfide, elemental sulfur (S), and sulfate (SO4). In the skeleton cycle shown in the chapter, sulfide is represented as the compound hydrogen sulfide (H2S) and sulfate is in the form of sulfuric acid (H2SO4). One prominent species of bacteria almost always at work in this part of the sulfur cycle is Desulfovibrio vulgaris. This bacterium has numerous “cousins” in genera with similar sounding names: for example, Desulfobulbus and Desulfobacter. Depending on the circumstances, two kinds of bacteria are concerned with reformation of sulfate. Under anaerobic conditions, the transformation of H2S to sulfur and sulfate is accomplished by photosynthetic bacteria, pigmented organisms (containing chlorophyll) that use light energy to replenish their ATP supplies. Other kinds of bacteria involved in recycling sulfur are aerobic, with the genus Thiobacillus being particularly noteworthy. The production of sulfuric acid from sulfide by thiobacilli can, however, have deleterious consequences.

Ecology is the branch of biology that deals with the relationships of organisms to one another and to their surroundings. In comparison with other kinds of living creatures, microbes are extraordinary with respect to the great diversity of ecological niches in which different species can grow. In other words, in the microbial world a particularly wide range of chemical and physical conditions can be tolerated and exploited. Some microbes, however, have the capacity to grow in solutions that contain very high concentrations of salts or other small molecules. These are known as osmophiles or halophiles (halos is Greek for “salt”). Certain microbial species called acidophiles are well adapted to life in acidic environments. Magnetotactic bacteria behave as bar magnets because each cell contains either one or two chains of magnetite particles. Further studies on magnetotactic bacteria may help to explain the purpose of magnetite throughout the animal kingdom. Looking for microbes in deep subsurface locales is usually approached by examining cores obtained by drilling from the surface. Recent reports indicate the presence of microbes in the pores of rocks that are deep below the Earth's surface (as far as 400 meters below ground). The symbiotic N2 fixation system of legumes is one example of microbes living in close association with higher organisms. Chemical "communication" between different species of microbes is an inherent feature of the carbon, nitrogen, and sulfur cycles. Life in a consortium is very economical and also greatly reduces dependence on environmental supplies of crucial nutrients.

In real life, more complex multicellular microbes also play major roles, for example in the chemistry of the environment and as pathogenic (disease-producing) agents. Fungi (singular, fungus) are particularly significant. They are eukaryotes and can be classified as follows: microscopic, and macroscopic (multicellular). Two basic features are common to multicellular microfungi: (i) under the microscope, cells are observed to occur in the form of threadlike filaments that often have branches, and (ii) the filamentous cell masses produce special reproductive structures that shed spores in great abundance. These features are illustrated by the scanning electron micrograph of the mould Penicillium shown in this chapter. Fungi are commonly observed growing in colorful patches on tree trunks or barren materials such as bare rocks and house roofs. A number of fungi are pathogenic for animals and plants. Fungal infections of humans and other animals are usually called mycoses. Important examples include thrush, diaper rash, vaginitis, and the lung diseases coccidioidomycosis (caused by Coccidioides immitis) and histoplasmosis (Histoplasma capsulatum). The ability of many fungi to grow on materials of low nutrient content explains why they frequently accumulate in buildings, causing nasal and eye irritation or respiratory distress to many people. As viewed under the microscope, Phytophthora infestans shows the typical features of eukaryotic microscopic fungi. Owing to an obscure technicality, it is now frequently referred to as "fungus-like". Leaf-cutter ants in tropical forests apparently invented agriculture long before humans did.

The “energy dynamo” of living cells can be roughly compared to a banking system in which different kinds of valuable materials can be transformed into units of the same kind of “energy currency,” adenosine triphosphate (ATP). From the viewpoint of bioenergetics, all (nonphotosynthetic) anaerobes behave in essentially the same way. The concept that sunlight is required for the growth of plants was not firmly established until 1779, but it was dimly foreseen by Stephen Hales, the acknowledged founder of plant physiology and one of the most prominent English scientists of the mid-18th century. Details of the pathway of carbon transformations in photosynthesis are not particularly relevant here, but one aspect is noteworthy: the means by which energy is circulated in the internal workings of photosynthesis. The O2-producing variety is found in microalgae and in bacteria called cyanobacteria (previously, but incorrectly called blue-green algae because of the color of their pigments), whereas the non-oxygenic type of photosynthesis is characteristic of the bacteria that is referred to as purple bacteria. Photosynthetic growth on organic compounds is described as “photoheterotrophic,” which simply means that light is used only as the source of energy for regeneration of ATP, and organic substances from the growth medium furnish the building blocks for biosynthesis of new cell materials. The similarities and differences between the cyanobacteria and the purple bacteria evoke many questions, particularly about biochemical and cellular evolution.

Vitamins deserve special attention for several reasons. They are chemical substances required for normal functions in all cells and organisms, from microbes to humans. The remarkable aspect of vitamin function is that only very small quantities are needed for normal growth and maintenance of living cells. The word vitamin was coined in 1912 to describe substances that were thought to belong to a category of organic compounds called amines that were vital for survival of certain microbes and health in humans and various animals. This resulted in the term vit-amines or simply "vitamins. It turned out that as more and more vitamins were discovered and characterized, some of them were actually not amines, but the name stuck. Vitamins are organic compounds of relatively small size (as compared with macromolecules such as proteins). As far as is known, all vitamins perform their vital functions in association with particular enzymes that are essential for normal metabolism. Vitamins form parts of various coenzymes and since these coenzymes are, in turn, parts of (catalytic) enzymes, it follows that vitamins must also act in "catalytic quantities". This explains why vitamins are required in only trace amounts. The principle of vitamin action is illustrated in this chapter. The function of vitamins can perhaps be made clearer by a concrete example. The B-vitamin niacin is an excellent case in point. The human body is unable to synthesize niacin or niacinamide, and therefore we must obtain this B-vitamin from animal, plant, or microbial foods that we consume.

This chapter talks about microbes that play an essential role in water purification and sewage treatment. Sewage presents several kinds of community problems. Certain kinds of disease-producing microbes can be transmitted via sewage. The ultimate purpose of sewage treatment facilities is to treat polluted water so that in the shortest possible time it is converted to “pure” water suitable for human use, or at least pure enough to put into the ocean or a nearby river without polluting it. This is accomplished in part by intensifying the activities of the assemblage of microbes that normally mineralize organic substances in the natural environment. If the amount of oxygen available is too low, foul odors develop due to hydrogen sulfide and noxious organic compounds produced by various microbes under anaerobic conditions. The self-purification of a river system is due primarily to the metabolic activities of bacteria, and the aim of sewage treatment disposal plants is to accelerate these activities under controlled conditions. In conventional sewage treatment systems, most of the original nitrogen and phosphorus atoms leave with the final effluent in the forms of nitrate, or ammonia, and phosphate. These inorganic nutrients can cause problems in the waters that receive the sewage plant effluent, for example, by encouraging eutrophication (nutrient enrichment of natural waters, frequently leading to excessive growth of algae). There are ways of removing these inorganic nutrients from the final effluent (so-called polishing treatments), but they are not in widespread use as yet.

This chapter deals with the history of communicable disease in the ancient world and with several pioneers of infectious disease research. Throughout the ages, human communities have been subject to the onset of devastating plagues and pestilences. Major episodes are described in considerable detail in numerous records of the past, as far back as 3000 B.C., so it is no surprise that the Bible refers to many different diseases. Reliable historical accounts detail the ravages of pestilences and plagues over the past two thousand years. The plagues and pestilences were, of course, caused by microscopic parasites such as bacteria, fungi, and other "invisible" biological agents. Before 1676 the existence of invisible microbes was undreamt of, and the idea would have been considered a fantastic notion to most people. How then did the germ theory of infectious disease originate and develop? The usual, and erroneous, answer is that the theory was the creation of Louis Pasteur. Students of the history of biological science are taught that the first really perceptive insights into the nature of infectious disease were advanced by the Italian Girolamo Fracastoro (ca. 1478–1553). The plague is an infectious disease that can take several forms, depending on the properties of the particular strain of the causative microbe. If the bacteria localize in the lung, the disease is called pneumonic plague.

Louis Pasteur's researches focused, among other things, on infectious diseases, at first on diseases of silkworms. It is ironic that Pasteur's misinterpretation of certain observations led him temporarily astray as to whether or not one of the apparently infectious diseases of silkworms was in fact caused by a microbe. In any event, this research was Pasteur's introduction to later study of infectious disease in more highly evolved domestic animals and humans. Robert Koch, 21 years younger than Pasteur, was combative as was Pasteur, and in some ways excelled Pasteur as an experimenter, at least in bacteriology. Koch isolated the bacterium Bacillus anthracis from diseased animals in pure culture and showed by the most rigorous criteria that this organism was the causative agent of anthrax. Koch later isolated the bacteria that cause tuberculosis (1882) and cholera (1883). The last phase of Pasteur's meteoric career was concerned primarily with prophylaxis against infectious disease, in particular by vaccination procedures. This was not a new concept; inoculation to induce immunity to smallpox had been practiced for centuries. The inoculation procedure worked well most of the time, but there were occasional failures. The method was totally empirical, and sometimes the child would actually become ill with smallpox. This happened to the young Edward Jenner (1749–1823) during a severe epidemic in England. He recovered and was thereafter immune to the disease, which became a definite advantage in his later work.

Pathogenic microbes gain entrance to the body in characteristic ways, depending on the microbe. The portal of entry for the bacteria that cause typhoid and paratyphoid fevers, dysentery, and cholera is the digestive tract. An important aspect of defense against microbial infection is the activity of special phagocytic cells that are very mobile and can engulf and destroy microbes. Individuals with phagocytes that do not function properly may have recurrent infections caused by microbes that are normally not pathogenic, and some forms of "phagocyte cell disease" are often fatal in childhood. The human immunodeficiency virus (HIV) attacks the host’s immune system, causing its eventual failure. This failure leaves affected individuals vulnerable to many infections and cancers, leading inexorably to severe morbidity and high mortality. Substantial evidence suggests that HIV emerged in the middle of the twentieth century, following the infection of humans with simian immunodeficiency viruses. Disparities in risk of infection and in access to treatment expose critical inequities in the distribution of social and medical resources within developed and developing countries. Theodore Rosebury made extensive studies on the aspect of microbiology and summarizes the overall microbial ecology of humans. Germ-free experimental animals become very ill, and may even die, if their diets are not supplemented with certain vitamins that do not have to be added to the diets of normal animals. This indicates that in normal animals, the microbial population in the intestine must furnish vitamins to the host.

Viruses are incapable of multiplying by themselves. Pioneering studies on viruses that attack bacteria laid the groundwork for analyzing details of the mechanisms employed by viruses that invade and multiply in plant and animal cells. A detailed knowledge of the structures of many viruses and of their replication processes is now present. After the end of World War II, microbiologists were finally beginning to see what viruses actually looked like, and they were indeed interesting: geometrical structures, of different degrees of complexity, approximately 10 to 100 times smaller than bacteria. The great breakthroughs in understanding of viruses came from study of bacteriophages, viruses that attack bacteria. Bacteria grow much more rapidly than animal and plant cells, and it is consequently much simpler to do many kinds of experiments with bacteria and with the viruses that attack them. The fundamental aspects of virus multiplication turned out to be essentially the same for bacterial, plant, and animal viruses. There are many differences in details that are important for understanding how to combat infectious plant and animal viruses, but the basic features of virus multiplication were first revealed most clearly by studying bacteriophages. The features that distinguish viruses from all other living organisms are presented in this chapter. Viruses lack the biochemical mechanisms needed for their own multiplication. In this sense, they could be called incomplete organisms. Viruses are inert and can be stored for long periods without loss of infectivity.

A variety of procedures are used to kill or inhibit the growth of potentially pathogenic microbes in food and drink, on objects that we contact daily, and on or in the body. The antimicrobial weapons now available are of two general kinds: physical agents and chemical agents. Heat is the most common physical agent used for killing microbes in food and on objects (sterilization). Pasteurization is aimed at killing the relatively heat-sensitive pathogenic bacteria likely to contaminate milk and other liquids; not all the microbes present are killed. Numerous classes of chemical substances can inhibit the growth rates of microbes or can kill them. Certain effective chemical agents can be used externally, on the skin or to disinfect objects. Others, such as antimetabolites and antibiotics, are used internally in order to fight disease. Antiseptics are chemicals that can be safely applied to the skin or mucous membranes. Disinfectants are chemical agents used to kill microbes in or on inanimate objects or materials. Antimetabolites ordinarily do not kill microbes, but they can slow down their growth rates drastically. The so-called sulfa drugs, widely used at one time for treating infectious diseases, are examples of antimetabolites. At present, interferons are probably the most potentially promising agents for treatment of virus diseases.

This chapter talks about how DNA performs its important role and how it can be used for genetic engineering. Some understanding of DNA structure is essential for even an elementary appreciation of the exquisite mechanisms involved in gene action. DNA is a large macromolecule composed of three kinds of chemical units that are arranged in a very specific manner. Two of these units provide the backbone of DNA in the form of a two-stranded helix, in which two coiled fibers are connected. The two backbones of DNA are held together by pairs of nucleic acid bases. These bases represent the third kind of unit in DNA and consist of four types of small nitrogen-containing molecules that have distinctive chemical properties: adenine (A), thymine (T), guanosine (G), and cytosine (C). The identification of DNA as the genetic material in 19443 and the rapid development of bacterial genetics starting about 1950 were the beginnings of a great new wave of discoveries that opened unexpected vistas in microbial biotechnology. There is general agreement that recombinant DNA technology is capable of producing many new and useful drugs, industrial solvents, fertilizers, and so on. Determination of the complete genome sequences of many different kinds of microbes has another important purpose, namely, to aid in analysis of the course of evolution of life on Earth. Detailed comparisons of the DNA base sequences in microbes of diverse physiological capabilities are certain to reveal much about their evolutionary relationships.

The Earth itself is about 4.6 billion years old, and several lines of evidence indicate that life on Earth began approximately 3.5 billion years ago in the form of anaerobic bacteria. Tracing the evolution of higher forms is aided greatly by the study of fossils, but fossils of early microbes are very rare and hard to find. The origin of life on Earth is one of the major unsolved mysteries of science. The first organisms on Earth were anaerobic prokaryotes, and these were the only forms of life on the planet for a very long period. Some investigators looked into hypothetical microbial evolutionary trees based on comparing the structures of genes that code for enzyme proteins required for normal metabolic processes. A major complication has arisen in that new evidence shows that during evolution, extensive transfer of genes (or parts of genes) has occurred among many bacterial species. This phenomenon, known as horizontal gene transfer (HGT), suggests that many, if not most, of the extant bacterial species are genetic chimeras. Since the first complete DNA sequence of a free-living organism, Haemophilus influenzae, was determined in 1995, literally dozens of other genomes, both prokaryotic and eukaryotic, have been investigated and mapped. We can expect that detailed knowledge of the genomes of a wide variety of organisms will soon provide important insights on how early life forms diversified through the ages.